If the aliens in the movie Signs could be poisoned by
why didn't the atmosphere kill them? Why would it have to be in liquid form?
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The aliens in this movie were not poisoned, per se by water. They instead acted as if they had been burned by a strong acid. Water vapor may have a similar effect but only if they are exposed to it long term. Depending on the water density in the atmosphere, it may act mostly as an irritant in the short term.
Water COULD function as an acid, but it requires a number of conditions to be met. It is a rather simplistic idea that, because the pH of water is 7, it is "neutral." It isn't.
Water has 2 lone pairs of electrons on the oxygen, one of which can be donated to a suitable acceptor. Water is therefore a Lewis base.
Water also contains hydrogens, which under suitable conditions can be donated as protons. Therefore, water is a potential Bronsted-Lowry acid.
In many environments, it will act as neither. But in the right environment, with suitable reagents, it can act as either
Explaining this without invoking acidity/basicity:
In the presence of strong hydrides, water acts as an acid:
In the presence of strong acids, water acts as a base:
Water can act as either an acid or a base, dependent on the circumstances.
The movie isn't an alien invasion movie; it's a movie about faith and redemption. It's the story of the lost and angry ex-Father Graham Hess' road to forgiveness and regaining the strength to be a father, brother and preacher and, most importantly, to accepting his wife's death and forgiving her killer.
The aliens are an allegory for Graham's demons and they may not even be objectively real.
It isn't that water harms them, it's Graham's belief that the water harms them that harms them.